Horse Anatomy

The term anatomy means the study of the structure of a particular species and all its individual components. A horse's anatomy consists of several different body systems that work in combination to maintain all normal body functions.

Each body system plays a crucial role in keeping our horses healthy and happy while living their daily lives.

These body systems include:

• Skin

• Musculoskeletal

• Digestive

• Nervous

• Circulatory

• Immune

• Endocrine

• Respiratory

• Urinary

• Reproductive

Skin Anatomy in Horses

The largest organ of the horse is the skin. It makes up about 12–24% of the horse's body weight. It consists of four distinct areas:

• Haired

• Non-haired

• Pigmented

• Non-pigmented

Skin can be divided into three main layers: epidermis or uttermost layer, dermis or middle layer, and the subcutis or innermost layer. The main functions of the skin include being a protective barrier against the environment, regulating temperature, and providing the horse with a sense of touch.

Other important skin components include hooves, hair, subcutaneous muscle, and fat.

With skin often being the first line of defense against the weather, insects, and other toxins, it is often under attack but rarely leads to any life-threatening conditions.

Some of the common skin conditions seen in horses include:

• Dermatitis

• Congenital or inherited skin disorders

• Epitheliogenesis imperfecta

• Lethal White syndrome

• Arabian Fading Syndrome

• Hereditary Equine Regional Dermal Asthenia (HERDA)

• Allergies

• Rain rot

• Hair loss (alopecia)

• Mange

• Lice

• Dandruff

• Cutaneous habronemiasis (summer sores)

• Biting insects (including files, mosquitos, ticks)

• Hives

• Photosensitization

• Ringworm

• Tumors

• Cellulitis

Musculoskeletal System in Horses

The musculoskeletal system in a horse includes bones, cartilage, muscles, ligaments, joints, tendons and connective tissues.

This system supports the body structure, allows movement and protects the vital organs. While this system is what allows horses to be athletes it is also commonly a system that is prone to disorders and injuries.

Horses are expected to work, perform, and carry a rider which often puts stress on the muscles and bones. Disorders of the musculoskeletal components often lead to lameness.

Functions of each individual structure within the musculoskeletal system include:

• Bones: provide a rigid structure to the body and help to protect and shield the organs from any damage.

• Joints: where bones come together. Joints help a horse to have motion and can allow for rotation, bending, straightening or may not move at all.

• Cartilage: a smooth protective tissue on the ends of bones that help to reduce friction as joints move.

• Muscles: there are two types of muscles within the musculoskeletal system: skeletal and smooth.

  • Skeletal muscles: responsible for movement and posture

  • Smooth muscles: facilitate involuntary processes in the body such as movement of food along the digestive tract.

• Tendons: a type of connective tissue made up mostly of a protein called collagen that connects the end of a muscle to a bone.

• Ligaments: a type of connective tissue that can stretch and surround joints to help support and stabilize them. Ligaments can also connect bone to bone.

Common disorders of bones and joints include:

• Birth defects

  • Angular limb deformities

  • Dwarfism

  • Defects of the spine

• Developmental diseases

  • Osteochondrosis (OCD lesions)

  • Physitis

  • Contracted tendons (club foot, knuckling, flexural deformities)

• Joint disorders

  • Arthritis

  • Bursitis

• Disorders that can happen across the musculoskeletal system

  • Fractures

  • Tendonitis

  • Bursitis

  • Arthritis

• Disorders specific to the foot

  • Sole bruise or abscess

  • Canker

  • Contracted heels

  • Fracture of coffin bone

  • Keratoma

  • Laminitis

  • Navicular disease

  • Pedal osteitis

  • Puncture wounds

  • Heel crack

  • Scratches (pastern dermatitis)

  • White line disease

  • Sidebone

  • Thrush

• Disorders specific to the fetlock

  • Ringbone

  • Digital sheath tenosynovitis (windpuffs)

• Disorders of the carpus and metacarpus

  • Bucked Shins

  • Desmitis

  • Hygroma

  • Splints

  • Suspensory desmitis

• Disorders of the elbow and shoulder

  • Sweeney

• Disorders specific to the tarsus

  • Bog spavin (inflammation of the hock joint)

  • Bone spavin

  • Stringhalt

  • Thoroughpin

• Disorders specific to the stifle

  • Dislocation of the kneecap (patella)

  • Bone cyst

• Disorders specific to the hip

  • Dislocation

• Disorders specific to the back

  • Degenerative disease of the spine

  • Kissing spine syndrome

  • Sacroiliac injury

Muscle disorders of the horse include:

• Nutritional myopathies

  • Selenium deficiency

  • Vitamin E deficiency

• Exertional myopathies

  • Sporadic exertional rhabdomyolysis

  • Chronic exertional rhabdomyolysis

• Polysaccharide Storage Myopathy (1 and 2)

• Malignant hyperthermia

• Recurrent exertional rhabdomyolysis

  • Hyperkalemic periodic paralysis

• Infectious Myopathies

  • Virus-associated myositis

  • Sarcocystis myositis

  • Strangles-associated myositis

  • Clostridial myositis

  • Muscle abscesses

• Toxic myopathies

  • Ionophore toxicity

  • Plant intoxication

• Circulatory disturbances

• Fibrotic and ossifying myopathy in Quarter horses

• Immune-mediated myositis

Digestive System in Horses

The digestive system of the horse includes all the organs that are responsible for taking in and processing food. The structures involved include:

• Mouth

• Esophagus

• Stomach

• Liver

• Pancreas

• Intestines

• Rectum

• Anus

The digestive tract can be split into the foregut and the hindgut. The foregut is made up of the stomach and small intestines while the hindgut includes the cecum, large colon, and small colon.

The main functions of the digestive system consist of:

• Grabbing and chewing food

• Making saliva

• Drinking water

• Swallowing food and water

• Digesting food

• Absorbing nutrients

• Maintaining the proper balance of fluids and electrolytes such as salt

• Eliminating waste products

The digestive system is a common cause of concern in horses since it is an area prone to illness. When a horse's digestive tract is not working properly there are four main categories to consider: digestion, absorption of nutrients, motility (ability for food to move through the digestive tract) and elimination of feces.

Common digestive system disorders include:

• Choke

• Diarrhea

• Colic

• Anorexia (loss of appetite)

• Excessive drooling

• Parasites

• Viruses

• Potomac Horse Fever

• Bacterial infections

• Gastric ulcers

• Tumors

Nervous System in Horses

The horse's nervous system is made up of the brain, spinal cord and nerves found throughout the body. The nervous system allows the horse to experience and respond to sensations through a complex circuit that connects the entire body.

Reflexes are an important part of the nervous system that allow a horse, for example, to close its eye when the eyelid is touched.

There are many signs that can signify a sign of disease in the nervous system but the most common include:

• Behavioral changes

• Seizure

• Tremors

• Pain

• Numbness

• Lack of coordination

• Weakness or paralysis

Common neurologic disorders seen in horses include:

• Encephalitis

• Equine protozoal myeloencephalitis (EPM)

• Paralysis

• Parasites

• Neonatal encephalopathy in foals (neonatal maladjustment syndrome)

• Meningitis

• Rabies

• Botulism

• West Nile Virus

• Epilepsy

• Narcolepsy

• Stringhalt

• Equine laryngeal paralysis

• Polyneuritis equi

• Shaker foal syndrome

• Injury or trauma

• Wobbler syndrome (cervical spondylomyelopathy)

• Equine motor neuron disease

• Equine infectious anemia

• Equine herpesvirus

Circulatory or Cardiovascular System in Horses

The cardiovascular system of the horse is made up of the heart and blood vessels (veins and arteries). The main function of the heart is to pump blood to the body.

The heart functions can be split into two sides. The right side of the heart pumps unoxygenated blood to the lungs, where oxygen is added to the blood and carbon dioxide is removed. The left side of the heart pumps the oxygenated blood to the rest of the body to deliver nutrients to the tissues.

Common disorders of the cardiovascular system include:

• Heart failure

• Arrhythmias

• Blood clots

• Aneurysms

Immune System in Horses

A horse's immune system consists of white blood cells, antibodies, and other essential substances that help with fighting off infections and foreign invaders. There are also several organs involved in the immune system including:

• Thymus gland

• Bone marrow

• Spleen

• Lymph nodes

• Liver

The thymus gland and bone marrow are the location where white blood cells are produced while the spleen, lymph nodes, and liver trap microorganisms and foreign invaders. They also serve as a place for the immune cells to interact, collect, and generate an immune response when needed. The primary function of the immune system is to defend the body from foreign invaders or abnormal cells.

Common disorders involving the immune system seen in horses include:

• Anaphylactic shock

• Hives and swelling

• Heaves (recurrent airway obstruction)

• Sweet itch

• Vasculitis

• Purpura hemorrhagica

• Anterior uveitis

• Failure of passive transfer (in foals)

Endocrine System in Horses

The endocrine system of the horse consists of a group of tissues and organs that release hormones into the bloodstream. Most of the endocrine tissues consist of glands that release hormones directly into small vessels within or near the tissue. Other important organs that release hormones include the heart, kidney, and liver.

The horse's body uses a feedback system to monitor and adjust the level of each hormone throughout the body at any given time. Hormones are important as they help with functions such as temperature and blood sugar levels.

Common disease of the endocrine system seen in horses include:

• Cushing’s disease (PPID)

• Equine metabolic syndrome

• Granulosa theca cell tumor

Respiratory System in Horses

The respiratory system of a horse consists of the lungs as well as the large and small airways. As a horse breathes in, the air travels down the trachea, divides into tubes called the right and left bronchi, down small airways called the bronchioles and then into the lungs.

The main function of the respiratory system is to deliver oxygen to the blood and remove carbon dioxide from the body.

Common respiratory disorders of the horse:

• Pneumonia

• Diaphragmatic hernia

• Disorders of the larynx

• Disorders of the paranasal sinus (sinusitis, ethmoid hematoma, sinus cyst)

• Dorsal displacement of the soft palate

• Epiglottic entrapment

• Equine herpesvirus infections

• Equine influenza

• Hendra virus

• Equine viral arteritis

• Exercise-induced pulmonary hemorrhage

• Guttural pouch disease

• Inflammatory airway disease

• Lungworm infection

• Recurrent airway obstruction (heaves)

• Strangles

Urinary System in Horses

The urinary system of the horse consists of:

• Kidneys

• Ureters (tube that connects kidneys to bladder)

• Bladder

• Urethra

There are several important functions of the urinary system including:

• Excreting waste products

• Maintaining the correct balance of water and electrolytes in the body

• Production of hormones which are important to maintaining a healthy blood pressure,

• Producing blood cells

• Absorbing salts

• Processing vitamin D

Common urinary diseases in the horse:

• Bladder infections

• Bladder stones

• Kidney infection

• Kidney disease or failure

• Urinary stones

• Uroperitoneum in foals

• Patent urachus

• Tumors

• Leptospirosis

Reproductive System in Horses

The reproductive system in a horse functions to produce offspring. This system in both males and females is composed of primary sex organs and primary regulatory centers.

The primary sex organs of a male horse are the testes while the female has ovaries and a uterus. The primary regulatory center for both the male and female is in the brain and controls the production of hormones that influence the function of the sex organs.

Common reproductive disorders found in horses:

• Infertility

• Congenital and inherited diseases such as cryptorchidism

• Abortion

• Brucellosis

• Contagious equine metritis

• Equine coital exanthema

• Mastitis

• Endometritis

• Pyometra

• Placentitis

• Retained placenta

• Uterine prolapse

• Vulvitis and vaginitis

• Tumors

Horses maintain reproductive cycles that are important to consider when planning an equine pregnancy, especially if a horse owner plans to utilize artificial insemination. 

Equine Anatomy Considerations

Many of the disorders that have been mentioned for each anatomical system may affect more than one body system at a time. The best way to keep your horse’s body in ideal condition is to keep up with annual wellness veterinary care as well as farrier, vaccination, and deworming schedules. Wellness care is the best prevention we have to allow our horses to live long healthy lives.

Be sure to always be observant and listen to your horse if they are not acting normal and call your primary veterinarian with any changes or concerns.

Featured Image: iStock.com/Callipso